1. Field of the Invention
The present invention relates to a multipole lens, aberration corrector, and electron microscope.
2. Description of Related Art
In an electron microscope such as a transmission electron microscope (TEM) or a scanning electron microscope (SEM), spherical aberration and chromatic aberration are one type of factor leading to decreases in resolution. In an electron microscope, if spherical aberration or chromatic aberration is corrected, the resolution will be improved. In a spherical aberration corrector or in a chromatic aberration corrector, magnetic or electric fields are produced by the use of multipole elements to correct spherical aberration or chromatic aberration.
For example, in M. Haider, G. Braunshausen, E. Schwan: Optik 1995, No. 4, pp. 167-179, a multipole element producing only magnetic fields is disclosed as a multipole element used in a spherical aberration corrector, and this multipole element is fabricated by winding a coil on a dodecapole (12-pole) polepiece made of soft iron that is a soft magnetic material. In the technique of M. Haider, G. Braunshausen, E. Schwan: Optik 1995, No. 4, pp. 167-179, a liner tube that is a metal tube for evacuating the inside is placed in a central hole formed in the multipole element. Only the inside is evacuated. The multipole element itself is placed outside a vacuum. Therefore, an electron beam passes through the vacuum but a magnetic field produced by the multipole element leaks into the liner tube made of a nonmagnetic material, thus producing a desired symmetric field with respect to the electron beam.
“Nano Electron Optics”, Katsumi Ura, p. 284 also discloses a dodecapole element capable of producing static electric or magnetic fields. Each polepiece is made of a metal consisting of a soft magnetic material. A static magnetic field is generated by a coil. A static electric field can be produced at the same time by applying voltages electrically. The multipole element itself is placed in a vacuum. The electron beam passes through the center of the multipole element and is affected by a multipolar field including the static electric and magnetic fields.
In addition, JP-A-2010-114068 discloses a dodecapole element capable of producing static electric and magnetic fields in the same way as the multipole element discussed above. The multipole element disclosed in JP-A-2010-114068 is different from the multipole element disclosed in M. Haider, G. Braunshausen, E. Schwan: Optik 1995, No. 4, pp. 167-179 in that only the multipole element for producing static magnetic fields is placed in a vacuum and that the multipole element consisting of polepieces and coils for producing static electric fields is placed outside the vacuum.
The above-described multipole elements can produce only a static magnetic field or a field of superimposed static electric and magnetic fields having a constant intensity in the direction of motion of an electron beam. That is, the above-described multipole elements cannot produce static magnetic fields that are different in intensity relative to the direction of motion of an electron beam.